It is advantageous for a multiple chip device to have the following features:                The pressure distribution across each chip and between chips is uniform;        The applied pressure should be within a defined operating range.        
In traditional designs, semiconductor chips are placed between mechanical strain buffers, such as molybdenum or tungsten, to form a semiconductor unit. These semiconductor units are then placed between two common copper electrodes (see FIG. 6 for an illustration of this). For improved performance, in terms of current handling capability and reliability, the pressure distribution across the chips within the package should be as uniform as possible. This is difficult to achieve, as micron-level differences in semiconductor unit thickness or surface flatness variations across the copper electrodes can lead to applied pressures that differ greatly between semiconductor chips and also differ from the intended target pressure. The flatness and parallelism of components, such as heatsinks in the end-users' application will also have a significant effect on this. The effect is amplified in cases where multiple devices are clamped in a single stack for series operation, due to the additive effect of all of the tolerances within the stack. The inventor has appreciated that there are applications where up to twenty such devices are clamped in a single stack (see prior art 1 below).
It is known that the performance of semiconductor chips is affected by the level of applied pressure in pressure-contact applications, such that under- or over-pressurisation can result in sub-optimal performance and poor reliability. In addition to this, the pressure that can be applied to the chips within the traditional design is not limited in any way, so they are vulnerable to both under- and over-pressurisation by the end-user.
One approach (shown in prior art 2 below and also see FIG. 6) is to use extremely tightly toleranced components (typically matched to within 1 μm), to ensure component thicknesses are as closely matched as possible and to dictate that end-users provide clamp components (such as heatsinks and load spreaders) that have a much tighter flatness tolerance for multi-chip pressure contact devices than is used for traditional single chip pressure contact devices, such as large-area thyristors (e.g. typically 10 μm flatness tolerance, instead of 30 μm) (see prior art 3). Tight component tolerance ranges become hard to manage with large numbers of components, as is the case in large-area multi-chip pressure contact devices. Tight flatness tolerances for clamping components also become harder to achieve across a large surface area, compounding the problem.
Another approach is to use individual disc spring stacks in line with each individual semiconductor unit within the housing in order to reduce the force/displacement ratio. In this way, for a given difference in semiconductor unit thicknesses or a given flatness variation, the difference in contact pressure is minimised. As the disc springs are relatively poor electrical conductors, conductive metallic bypass strips (longitudinal current bypass) or stamped, contoured metal sheets (lateral current bypass) are used. Bypass strips run from the top to the bottom of the stack of disc springs. When the disc springs are compressed, the flexible bypass strip bows outwards. This arrangement is used in ABB's StakPak arrangement which is shown in FIG. 1. Due to the outward bow of the bypass strips, the packing density of chips may be limited, in turn limiting the current density of the finished device.
The stamped, contoured metal sheet is shaped as shown in FIG. 2 and incorporated in the finished device described in patent number CN103579165 (prior art 4). As with the longitudinal current bypass strip method in FIG. 1, due to the shaping of the sheet, and the area required to form the sheet, the packing density of chips is limited, again limiting the current density of the finished device.
A further approach is to use a pressurised fluid to pressurise the contact components in the device. This has been detailed in patent JP2004158489 (prior art 5). This does, however, rely on the availability of pressurised fluid, which is feasible in hybrid vehicles, but less so in typical industrial and transmission and distribution applications.
Two approaches for applying correct applied pressure are known. Both apply only to the design using disc springs described above.
The first approach (as used in ABB's StakPak) uses the rigid insulative sleeve of the device housing as a travel-stop mechanism, which prevents depression of the external contact surfaces beyond a defined plane (prior art 2). The chips are grouped into sub-modules which contain the spring assemblies. Either four or six sub-modules are used in a finished device, each of which is individually tested before being assembled. The device specification requires the load to be sufficient to compress the external contact surfaces level with the top of the sleeve (referred to as the threshold load in this document). Beyond the threshold load, any excess load is supported by the sleeve and the load applied to the chips is then dictated by the load/displacement ratio of the spring system and increases no further, even with extra loading. The displacement is defined as the difference between the pre-load height of the spring stack and the loaded height of the spring stack, once the sleeve begins to carry the mechanical load. In this design, only the sleeve supports load above the threshold load around the perimeter of the device, with no mechanical support provided in the centre of the device.
The second approach described in CN103579165 (prior art 4) uses a rigid insulative support frame inside the device housing to act as a travel stop mechanism. This acts in a similar manner to the perimeter travel stop mechanism described above. From the images given, it appears that load above the threshold load is supported in the centre of the device as well as at the edge. For this design to be tested or operated, the entire device must be fully assembled. Operation as a subassembly is not possible.
Investigations of pressure distribution using tightly toleranced components in a 125 mm press-pack package have found pressure uniformity to be very poor. Similar trials involving a 125 mm package may have also been found to have poor pressure distribution. Such difficulties achieving uniform clamping indicate that performance of the device as a whole is likely to be far from optimum. It has also been found that pressure distribution is greatly affected by the flatness of the clamping components that apply the mechanical load to the external pole faces of the device. Poor pressure distribution may have been seen even when using clamping components that were lapped flat and had measured flatness of around 5 μm across their entire device contact area, which is far flatter than called for in IXYS UK Westcode's application note in a press with adaptive heads, specially-designed to ensure parallelism between the upper and lower clamping surfaces.
Trends in patent activity over the past 25 years show a migration from rigid solutions for multi-chip pressure contact devices to solutions with enhanced compliance. These solutions include sprung solutions (ABB, Toshiba, Infineon, State Grid Corporation China) and solutions where the uniform pressure is provided by a pressurized fluid (Honda).
The only rigid solutions for which significant information has been found are those of IXYS UK Westcode, Toshiba and Fuji Electric. The Fuji Electric devices may not appear to be manufactured any longer. Little information can be found on the Toshiba devices, although they may be still advertised and sold. Recent published literature from IXYS UK Westcode large-area press-pack IGBT devices indicates that they may have reliability issues, which are suspected to result from pressure distribution problems (see prior art 6 and 7). These papers show that with a ΔTj of 78° C. the cycles to failure for an IXYS UK Westcode multi-chip pressure contact device is approximately 6,000 cycles for multi-chip pressure contact devices of rigid construction. This may compare unfavourably with single chip pressure contact devices, for which at a ΔTj of 80° C. the cycles to failure is far in excess of 100,000 cycles. Other published literature reviews have found by simulation that pressure uniformity may be grossly affected by micron-level differences in components in each chip's component stack.
Published literature is available on ABB's sprung solution, the StakPak, including case studies of HVDC Light schemes around the world using as many as 6,000 StakPak devices per scheme. One ABB presentation identifies 10 such schemes (prior art 1). According to ABB's own literature, their sprung solution has proven to be generally reliable in HVDC Light schemes. This published literature even provides details of failure rates, which appear to be relatively low.
It is desirable that to create high reliability multi-chip packages, a more compliant solution than that offered by the rigid copper electrodes is required.
A mechanical prototype of the design has been produced. As its design intended, the pressure uniformity was changed relative to that achieved with the traditional design device.
In addition to the background description above, we also summarise the general prior art as follows:    Prior Art 1—ABB (2014) High power semiconductors for T&D and industry application: StakPak & IGCT Introduction
Slide 18 shows 20 devices in one series stack.
Slide 19 states that by 2012 there were 10 HVDC Light projects using the ABB IGBT StakPak.    Prior Art 2—ABB (2015) StakPak: IGBT press-pack modules
Page 3 shows operation of ABB StakPak with loading stops.    Prior Art 3—IXYS UK Westcode (2015) Application note for device mounting instructions
Surface flatness instructions indicate a flatness tolerance of 10 μm for multi-chip pressure contact devices as opposed to 30 μm for standard single chip pressure contact devices.    Prior Art 4—Patent CN103579165—Full-pressure-welding power device—State Grid Corp China
An example of a compliance approach using disc springs within a ceramic housing.    Prior Art 5—Patent JP2004158489—Honda Motor Co Ltd
An example of a compliance approach using pressurised fluid to provide uniform loading to the pressure contact components within the device.    Prior Art 6—Tinschert et al (2015)—Possible Failure Modes in Press-Pack IGBTs
This paper investigates the poor reliability of traditional design press-pack IGBT devices. The authors conclude that the premature failures are the result of a mixture of over-pressurisation and under-pressurisation of certain chips in a device. Under-pressurisation particularly affects chips located at the edge of the device. This paper shows that with a ΔTj of 78° C. the cycles to failure is approximately 6,000 cycles for an IXYS UK Westcode multi-chip pressure contact devices of rigid construction. This compares unfavourably with single chip pressure contact devices, for which at a ΔTj of 80° C. the cycles to failure is far in excess of 100,000 cycles.    Prior Art 7—Frank (2014) Power Cycle Testing of Press-Pack IGBT Chips
This thesis details follow up work from the study by Tinschert et al (2015) (prior art 6). Individual press-pack IGBT chips are subjected to power cycling. The author found that individual chips have a lifetime that is orders of magnitude greater than fully-assembled devices.